Are those the numbers after you installed the Australian 90:1 transformers and removed the parallel resistance, along with the rest of the components of the hi-pass filter? If so, fear not. The efficiency is so much higher without the parallel resistor that I am sure 50W would do. My Atma-sphere MA240s don't make much more than 100W, and they are sailing with ease when I now run my 845PXs. In any case, and to answer your question, these amps are quite comfortable with those impedances; there may be a small loss in efficiency at 59 ohms, but it's nothing to worry about. (I will leave that to Ralph.) You're probably getting on average MORE power, lower distortion, wider bandwidth, at these impedances than what you can achieve into 8 ohms or certainly anything below 8 ohms.

The data you give sound like the measurements I made on my own 845s after I installed the AU transformers; perhaps you are reiterating the data I put up on SLOG.

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Lew I got a pr of the stock Acoustat interfaces...from a shop in town here an for a long time i have found the just runing the transfourmers..in to there own panels was the best sound i could get get...like the M3s has 3panels i would run the two out side panels of the base tranfourmer an the one panel in the middel of the high tranfourmer....with nothing in front on the primary side an nothing on the sucdary side...sounded great.....so now back to the stock set i got... Seeing how the high tranfourm did not over heat driveing it with the full input i though i give the stock set a try with the new setup you an others are doing with your SL....But just use the high tranfourmers so the setup is ....removed the parallel resistance, along with the rest of the components of the hi-pass filte in the c mod

Well just as you say the SLP went way up...i now have the best sounding highs i have ever got out of any of the Acoustat...this has alway be the down side to the acoustats for me littel to rolled down...but now i am driving the M3s with 60wats tube amps an thay sound great.... so... removed components of the hi-pass filte takes the BIGst lode of the Amp....the SL high fr thranfourm well give you even better highs than the New transfourmers you have put in...i cant try the SL i dont have any SL here ... just maybe someing you would like to try...

I would have never tryed this if i had not found this old pair of Acoustat interfaces just laying in the back... thay said there junk....i gave $100. for the pr...no panels in the deal but i have 8ea panels here so good to go ... thanks for your work an other on this type if setup. Goodluck

The credit should go to "Will" in Australia, aka "William" on the SLOG these days. He had the cojones to try removing all active and passive elements from the SL backplates and just run two audio transformers in parallel. I would never have thought it would work well, which is why I made my own measurements before even trying it. Then of course credit goes to Rob MacKinlay who actually builds that full-range 90:1 transformer. Crazy as he sounded, when Will first contacted me about his findings, I could not have been more ready to try a new approach, because I was thoroughly tired of having to use the Zero Autoformers and putting up with the pronounced midrange suck-out that I still could hear even with the Zeros in the circuit. Plus I knew intuitively that having the Zeros, the resistors, and the capacitors all in the audio path along with the bass and treble step-up transformers was not exactly the best way to maximize transparency. I was ready to consider buying another pair of speakers, which is part of the reason I bought those Beveridge 2SWs; had I known how well this new set-up with the 845PXs was going to work, I probably would not have done it.

Crazy as he sounded, when Will first contacted me about his findings, I could not have been more ready to try a new approach, because I was thoroughly tired of having to use the Zero Autoformers and putting up with the pronounced midrange suck-out that I still could hear even with the Zeros in the circuit. Plus I knew intuitively that having the Zeros, the resistors, and the capacitors all in the audio path along with the bass and treble step-up transformers was not exactly the best way to maximize transparency.

Thanks for telling how YOU relly feel.......this was how i felt when i had the SL A1 on the frist day...

As was said Lew has already used a 0.01uF teflon cap here instead of a 0.015uF cap without any problems. I will be using 0.0075uF teflon.

So if i am geting were you are now.....you are just using the two tranfourmers per speaker.....not the caps?

Also... on the Acoustat base tranfourm primary you can see there is a 1ohm 20w stock res. on the neg leg....in the Acoustats the base tranfourmer get hot if the one ohm removed....i have put a 2ohm 50watt..in an it sounds great.....This well work with the SL also.

The one thing i am finding with the new seup in my Acoustats.....if the base is driven into over loded... the highs an mids are never sucket out...i have never heard this befor... with any ESLs......i well saY with this setup the SPL is at 90-91...thats a Hell of big deal...

An i go out on the lim...say that it sounds to me like NO caps No coils...maybe just a fuse...should Ever be on the primary side of Any ESL tranfourmers....I well be moveing on to my martinlogans next.

In the setup now with My Acoustats on the secdary side i still have the 50k 100watts on the base...an have put in the .0015 6k oil caps in... were the Stock .01 ...thay are two high ...on the secdary side of the high frc.tranfourmer

None of us, not even Will, has deleted the mixer box, which as you know contains two capacitors, one each in series with each arm of the secondary of the treble transformer. I don't think it would be safe to do that. But as you note, I found and purchased some Russian 8kV .01uF teflon film caps on eBay, and I have used those to replace the stock .015uF caps that are used by SL. IMO, the Russian caps sound better than the OEM ones.

Also, Will deleted the inductor in series with the SL bass transformer; I did not do that. I use a 1.5mH 12ga copper Alphacore air core ribbon inductor on the input side of the SL bass transformer.

Thanks lew...it all good..you can were my time goes... just liveing with this everday... This is the type of sound that makes me get up in the middel of the night...to see if it still there....it been 4days now..... i still cant beleve it....

As you know the Acoustat panels are 9"x 48" so i can add as minny as i like...the base lode for the tranfourmers i have found is 3ea pr side... thay are so well made that thay dont even have to have a frame ...thay can just be setup ezely....i have found the best sound is with a flat setup...like the Quad 57s...so now i am driving this setup with the M60.. It well play as loude as if thay were Klipsch..go figg...like you sead in a post i read..this is the sound i been looking for for 40years...

So if we look at the Acoustat 121 interface in this setup the 50k on the base tranfourmer secdary ...is like your induckter....an i have now on the high frc. tranfourmer the two .0015 6k oil caps...were there was the stock .01 5k caps....

One thing you may tell your frinds on the soundlabe site is this setup well work with the stock high frc.tranfourmers...this would let them see what a great setup this is...then thay can get the new tranfourmers if thay feel that thay need them....to test this just put a fuse in the high frc primary side an uses a 3amp...i did this on my stock high frc. tranfourmer an did not kill one fuse....Great fun....60tube watts goodluck

So, before Will first contacted me by email to tell me about the Australian transformers, I was wondering whether I could even just remove that parallel resistor in the hi-pass filter for the OEM SL treble toroid. Dr. West had repeatedly warned against doing that, for reasons having to do with the toroid, but it was clear to me that the major problem with driving the SL speakers using my OTLs had to do with that resistor. In fact, Will reported to me that he did remove the parallel resistor successfully in his early experiments, and upon hearing that he got away with it, I did the same, while waiting for the new transformers to get to me from Australia. You are quite correct; it's a big improvement vs with the resistor in place. What I did also was to reduce the series capacitance to 22uF so as not to change the -3db point of the hi-pass filter. (Accounting for the intrinsic impedance of the speaker at the crossover point, 22uF gives about the same "knee" as does 36uF plus the 8-ohm parallel resistor.) I think Will completely removed that capacitor on the primary of the toroid; I cannot recall what he said about that. I did not try it.